Any good SF world must be understood as a possibility space. A universe which is characterized first and foremost by its physics:
By what is and isn't possible, and by how easy (or difficult) each of these non-impossible things are to do.
Once you have created the possibility space, you must extrapolate how the people in the world will exploit the possibilities. Because, you know, they're people. They are going to exploit it. Some of them, at least. That's what people are like.
I'll mostly be talking about science fiction in this post, but the principles I refer to are eminently applicable to fantasy as well.
